Fusarium Head Blight (FHB) or scab of wheat is a fungal disease of the genus Fusarium that affects crops such as wheat, oats, barley, rye, corn and some grasses. The disease is of great interest, as it can have significant effects on crop yields. For example, the damage caused by the fungus Fusarium graminearum on wheat can be enormous. In particular, during 1993, this fungus caused an estimated three billion dollar loss to farmers in North America alone. The main effects of FHB infection are reduced crop yield and grain quality. Furthermore, FHB also produces mycotoxins which makes contaminated grain unsuitable for human or livestock consumption, for example, causing livestock feeding problems, such as refusal of feed or vomiting.
In order for this disease to occur, a susceptible host must be present and the environmental conditions must be favourable for infection and disease development. Since the environmental conditions cannot be controlled on a large scale, incorporating resistance genes into adapted wheat varieties is the most effective, economic and environmentally safe means of controlling the disease.
Naturally-occurring FHB-resistant cultivars of wheat are known and have been reported as two to three independently segregating genes in some Chinese and Brazilian strains (Van Ginkel et al., 1996, Plant Disease 80:863). Unfortunately, these resistant cultivars have undesirable agronomic traits, such as small heads and late maturation, and are therefore of little commercial value. However, the resistant cultivars have been used as parents in breeding programs with elite lines (Bai and Shaner, 1994, Plant Disease 78:760-765), with limited success. This is due to the fact that the inheritance of resistance to FHB is quantitative and controlled by many additive genes. As a consequence, data elucidating both the number and location of these genes has been problematic.
Hexaploid wheat is comprised of three homoeologous genomes (A, B, and D) each having 7 chromosomes. Procunier et al. 1998 , 9.sup.th lnt. Wheat Gen.Symp., Sask. SK.3:143-147 reported that the D genome ( chromosomes D1 to D7) lacked any significant FHB resistance genes. This placed the resistance genes on the A or B genomes. Two genetic regions ( 3BS and 6BL) for FHB resistance were identified by Anderson et al., 1998, National Scab Forum, St. Paul, Minn. and a single region ( 5A or 6B) is associated with the phenotypic marker awnless and FHB resistance( Ban et al. 1997, Fifth European Fusarium Seminar, Szeged, Hungary). However, Buerstmayr has recently showed that chromosomes 6D, 6B, 5A 4D, and 7A contain the FHB resistance genes ( Buestmayr et al., 1999, Theor. Appl Genet. 98:76-85).
Clearly, identification of the chromosomal locations of the FHB resistance genes followed by molecular mapping of these genes would greatly facilitate breeding FHB resistant strains. Specifically, molecular marker based-assays could be done on seed material, thus eliminating the lengthy growing time for assaying adult plants. Furthermore, seed testing does not require growth cabinet space or a costly nursery. The markers would also allow for gene pyramiding of multiple, independent resistance genes into complex genetic backgrounds. Ideally, the markers should be cost effective, highly reliable and accurate and require only a few hours for testing.